1. Field of the Invention
This invention relates to a focusing screen for effecting focusing in a camera or the like.
2. Related Background Art
Various types of focusing screens for single-lens reflex cameras have conventionally been used. One type of focusing screen known is obtained in the following manner: the surface of a base material such as glass is roughened through graining to prepare a focusing-screen matrix, from which a mold is prepared. A focusing screen can be produced by transferring this surface roughness onto the surface of a plastic material. Apart from this, Japanese Patent Laid-Open No. 58-60642 discloses a focusing screen, in which the rugged portions on the grainy surface obtained through graining are processed into a spherical configuration, thereby attaining an improvement in diffusion characteristic.
These conventional focusing screens are estimated high in terms of the naturalness in the blur they provide. On the other hand, when used with a lens having a relatively low transmittancy, or, even with a high-transmittancy lens, when stopped down, these focusing screens exhibit a rough graininess on the screen surface as if interspersed with fine sand, resulting in a rather poor visibility. This is attributable to the fact that a surface obtained through graining or a surface obtained by surface-finishing such a surface a microscopic surface roughness, with both the grain size and the height thereof being irregular. Further, such a surface exhibits a very high degree of randomness in terms of grain arrangement.
Apart from this a focusing screen structure is proposed in Japanese Patent Laid-Open No. 55-90931, etc. In this structure, micro-lenses having a microscopic surface roughness (in terms of grain size and height) are arranged with perfect periodicity. This focusing screen exhibits no rough graininess, and provides a satisfactory level of brightness as well as excellent visibility. On the other hand, its perfectly periodical structure restricts the diffracted light in a particular direction corresponding to the order of diffraction, resulting in a striped blur, etc. Thus, the blur obtained with this focusing screen is rather unnatural and disagreeable.
Some of the methods of manufacturing focusing screens having such a microscopic surface roughness utilize the well-known technology of photolithography (Japanese Patent Laid-Open No. 54-83846, etc.). In one of such methods, a master masking plate having a microscopic pattern is set in close proximity to or in close contact with the surface of a photosensitive-material layer formed on a substrate. In this condition, the photosensitive material is exposed, thereby forming on the photosensitive-material surface a microscopic surface roughness corresponding to the microscopic pattern on the master masking plate. In another method of this type, the exposed portion is completely removed, and a microsocopic surface roughness corresponding to the microscopic pattern is formed on the substrate surface by etching. Taking the well-known influence of the proximity exposure into consideration, the former method is superior to the latter. The reason for this assertion will be explained below.
In the proximity exposure method, exposure is effected using, for example, a master masking plate having a microscopic surface pattern as shown in FIG. 20 and a substrate whose surface is coated with a photosensitive material and which is spaced away from the master masking plate by a distance .DELTA.d, as shown in FIG. 19. Thus, even when using the same master masking plate, different illuminance distributions can be realized on the photosensitive-material surface by varying .DELTA.d. The influence of this variation in the illuminance distribution on the microscopic surface roughness is far greater in the former method than in the latter. This is why the former method is regarded superior to the latter.
This proximity exposure method, however, has the following problem: assuming that a microscopic surface roughness as shown in FIG. 1A is formed when .DELTA.d=0, i.e., in the case of close-contact exposure, an increase in the distance .DELTA.d results in this surface roughness being changed to a microscopic surface roughness as shown in FIG. 1B. As the distance .DELTA.d increases, the microscopic surface roughness undergoes a change as shown in FIGS. 1C and 1D. The relationship between microscopic surface roughness and diffusion characteristic is subtle. Generally speaking, when the flat portion in the microscopic surface roughness has a relatively large area, for when the difference between vertex and bottom in the microscopic surface roughness is relatively small, the quantity of the small-angle diffused light is large. However, as can be seen from FIGS. 1A to 1D, when .DELTA.d is small, the difference between vertex and bottom is large but, at the same time, the area of the flat portion is also large, and, by augmenting .DELTA.d, the area of the flat portion can be reduced but, at the same time, the difference between vertex and bottom is also diminished. As is apparent from the above, the proximity exposure method involves a large amount of small-angle diffused light, which makes focusing rather difficult.
This photolithography method can also be applied to the manufacture of a focusing screen having a regular surface roughness as mentioned above by using a master masking plate having a regular microscopic pattern (U.S. Pat. No. 4,567,123). As stated above, however, such a focusing screen with a regular microscopic pattern can only provide rather unnatural blur.
As stated above, an excessive degree of irregularity in the micrsocopic surface roughness of a focusing screen results in a rough graininess, and an excessive degree of regularity in the same results in an unnatural blur. In view of this, methods have been proposed according to which the microscopic surface roughness is endowed with a semi-regularity.
One of such methods is disclosed in Japanese Patent Laid-Open No. 59-208536. This method employs a master masking plate obtained by enlarging by the step-and-repeat method, the area of a reticule pattern with dotted figures in a semi-regular arrangement. However, the step-and-repeat method inevitably involves the problem of joints. If the problem of joints is overcome successfully, there still remains the problem that the macroscopic light and shade or sparseness and denseness in the reticule pattern becomes generally conspicuous when the area of the pattern is enlarged. Furthermore, in the method disclosed in this laid-open patent Application, the dot diameter, the dot center positions, and the inter-dot-center distance in the reticule pattern are determined totally at random by random numbers, which means the above-mentioned rough graininess is likely to appear due to the excessive degree of randomness.
The above problem is solved by a method disclosed in Japanese Patent Ladi-Open No. 63-221329 by the inventor of the present invention, which adopts a semi-regularity in terms of the microscopic surface roughness. This method aims to impart an appropriate degree of randomness to the arrangement of a regular microscopic surface roughness. In this manufacture method, which adopts the photolithography method, a master masking plate is used, on which figures having substantially the same configuration are arranged with a certain degree of variation in terms of their respective center positions with respect to regular periodical lattice points. This method involves no rough graininess due to an excessive degree of randomness. Further, the pattern drawing on such a master masking plate is performed in a state enlarged by several or several tens of times using an electron beam, a laser pattern generator, etc. Aferwards, the pattern is reduced to the master-plate size of approx. 50 mm.times.50 mm through reduction exposure. A range of this order, however, allows the pattern drawing to be completed at one time. If not, there is no need for performing step-and-repeat since this method is strictly limited to a regular arrangement of periodical lattice points and allows the drawing to be divided into several stages.
However, a focusing screen obtained by the above-described random-arrangement method involves the following problem: with such a focusing screen, a diffracted ray light of a higher order can be easily eliminated, whereas it is rather difficult to eliminate a diffracted ray of light of a lower order with such a focusing screen. The above-mentioned unnaturalness in the blur is attributable to the diffracted rays of light. In addition, the random arrangement causes sparse and dense areas to be produced in the microscopic surface roughness as shown in FIG. 2. In this dense areas, no difference between vertex and bottom exists in the microscopic surface roughness, whereas the sparce areas contain a relatively large amount of flat portion, with the result that the small-angle diffused light increases, as stated in connection with the proximity exposure method.